1. Field of the Invention
The present invention relates to the art of polyphase electric motors having a multipole cylindrical magnet.
2. Description of the Related Art
These two-phase or three-phase motors have the feature of a structure that is heteropolar on the rotor (annular or disk-shaped magnets having N pairs of poles) and heteropolar on the stator, with P teeth. N and P are chosen in such a way as to minimize the zero-current torque.
In particular, from the prior art there is known French Patent FR2742940, which describes a two-phase motor, especially a clockwork motor or a motor for driving a pointer of a display device. That patent describes a polyphase motor formed by a stator part excited by electric coils and by a magnetized rotor having N pairs of poles magnetized radially in alternate directions, N being equal to 3 or 5. The stator part has at least two W-shaped circuits, each containing an electric coil surrounding the central leg. The W-shaped circuits are disposed in such a way that, when one of the central legs is located opposite a magnetic transition, the other central leg is located opposite a magnetic pole, the pole shoes of the legs of a W-shaped circuit being spaced at angles of π/4 and the pole shoes of the central legs of two W-shaped circuits belonging to different phases being spaced at angles substantially equal to π/2±k.π/N, where N is the number of pairs of magnetic poles, or in other words 3 or 5, and k is equal to 0, 1 or 2.
French Patent FR2754953 describes a polyphase motor comprising a stator part excited by electric coils and a magnetized rotor having N pairs of poles magnetized radially in alternate direction, N being equal to 4 or 5. The stator part has at least two W-shaped circuits, each containing an electric coil surrounding the central leg. The W-shaped circuits are disposed in such a way that, when one of the central legs is located opposite a magnetic transition, the other central leg is located opposite a magnetic pole. The pole shoes of the central legs of two W-shaped circuits belonging to different phases are disposed at an angular spacing substantially equal to 120°.
The problem posed by the prior art motors is that of space requirement along the axis of rotation of the motor because of the use of cylindrical magnets, and that of the quality of the cylindrical magnets having radial anisotropy, which at equal energy cost three times more than disk-shaped magnets having axial anisotropy.